What everyone else said plus you have to return any borrowed book before you can borrow another one. So even though you will be eligible to borrow another book on January 1st you cannot do so until you return the one you currently have.

Hint: Since you can only borrow one book per calendar month, be sure it is worth borrowing. View the sample first. I made the mistake of borrowing a couple of books in the past few months that were gawd awful. Now I check them out via the sample before borrowing to hopefully weed out any stinkers.